The present invention was conceived in the course of studying problems encountered in attempting to verify the water output capacity of fire department pumper units and fire hydrants. State governments require periodic certification of fire department pumper units to insure compliance with state regulations and specifications and the capability of the unit to respond to emergencies. Certification of fire hydrant outputs is likewise required for insurance rating purposes. Certification requires very precise and accurate measurement of the water flow, usually within an accuracy of plus or minus one percent.
In order to accurately measure fluid flow through a conduit, it is desirable to have a substantially flat face velocity profile of maximum mean flow diameter i.e., a diameter approaching or approximating the inner diameter of the conduit. When the face of the velocity profile becomes concave or convex, the mean flow diameter decreases, rendering it more difficult to secure an accurate flow measurement, because the flow is not uniform over the cross sectional area of the conduit.
If a nonuniform flow profile is symmetrical relative to the axis of the conduit, use of an impeller type fluid flowmeter having the axis of impeller rotation coaxial with the conduit may be useful, because the hub of the impeller obstructs the core of the fluid flow and aids in establishing a somewhat more uniform flow through the vanes of the impeller, i.e., through the annular space between the impeller hub and the inner wall of the conduit.
However, if the flow profile is not axially symmetrical and/or the core flow is distorted, a propeller or turbine type flowmeter cannot provide an accurate measurement. Moreover, the reduction in the cross sectional area of the flow path caused by the propeller hub creates backflow at the margins and the mean flow diameter becomes unstable and highly distorted making it very difficult to measure flow velocity.
The condition becomes even more exaggerated if the fluid flow is swirling, i.e., traveling in a helical or corkscrew path through the conduit. With a swirl condition, the flow is distorted and the mean flow diameter is greatly reduced and far off center relative to the axis of the conduit, rendering flow measurement highly inaccurate.
All of these conditions are encountered in fire department verification and certification studies. Water flow from fire hydrants and fire department pumper units is usually highly distorted and has a major swirl factor. Similarly, a curve in a fire hose will distort the flow and impart a swirl to the flow, either a left hand swirl or a right hand swirl depending upon the direction of the curve in the path of water travel. Consequently, verification measurements of water output capacity and flow rates have required highly sophisticated equipment, and have been time consuming and expensive. Simpler, more reliable, more accurate and less expensive verification systems have been sought.
Impeller type rotary flowmeters are highly regarded in the flow measurement industry and usually provide a reliable and particularly economical system for measurement of fluid flow. It would be beneficial if the same could be used with a higher level of accuracy and reliability in conducting verification and certification studies, especially when encountering problematic flow conditions such as those above discussed.
The inventor herein named has previously developed a number of impeller or propeller type rotary flow-meters which are made and sold by the McCrometer Division of Ketema, Inc., Hemet, Calif. Some of these meters include flow straighteners adjacent the propeller for mitigating swirl in the fluid flow as the fluid passes the impeller, for example, longitudinally extending tube bundles or angularly spaced longitudinally extending vanes. See for example U.S. Pat. No. 3,049,009 for a disclosure of flow straightening vanes conformed to the frontal face of an impeller. Fire hydrant water meters containing such flow straighteners tend to reduce the swirl of the water emanating from the hydrant, but measurement of water consumption is not sufficiently reliable to be used for purposes of verification and certification.
The inventor herein has also developed a static device which has been used for measuring fluid flow, see U.S. Pat. No. 4,638,672, and for mixing diverse fluids, see U.S. Pat. No. 4,812,049. Fluid flowmeters and static mixers embodying the patented device are made and sold by the McCrometer Division of Ketema, Inc. under the registered trademark "V-CONE".